The invention relates to the field of data compression and most closely relates to real time compression of a digitally sampled audio data stream.
The invention relates to a data compression apparatus for data compressing an audio signal, to a data including compression method, a transmitter including the data compression apparatus, a recording apparatus including the data compression apparatus, a record carrier having the data compressed audio signal recorded on it in a track of the record carrier, to a data expansion apparatus for data expanding a data compressed audio signal, to a data expansion method a receiver including the data expansion apparatus and to a reproducing apparatus including the data expansion apparatus.
Data compression on an audio signal is well known in the art. Reference is made in this respect to EP-A 402,973, document D1 herein. The document describes a subband coder, in which an audio signal is A/D converted with a specific sampling frequency, such as 44.1 kHz, and the resulting samples in the form of eg. 24 bits wide words of the audio signal, are supplied to a subband splitter filter. The subband splitter filter splits the wideband digital audio signal into a plurality of relatively narrow band subband signals. Using a psycho acoustic model, a masked threshold is derived and blocks of samples of the subband signals are subsequently quantised with a specific number of bits per sample for each block of the subband signals, in response to the masked threshold, resulting in a significant data compression of the audio signal to be transmitted. The data compression carried out is based on xe2x80x98throwing awayxe2x80x99 those components in the audio signal that are inaudible and is thus a lossy compression method. The data compression described in document D1 is a rather intelligent data compression method and requires a substantial number of gates or instructions, when embodied either in hardware or software, so that it is expensive. Moreover, the subsequent expansion apparatus also requires a substantial number of gates or instructions, when implemented in hardware or software-respectively.
Those skilled in the art are hereby referred to the following listed documents:
(D2) xe2x80x9cA digital decimating filter for analog-to-digital conversion of hi-fi audio signalsxe2x80x9d, by J. J. van der Kam in Philips Tech. Rev. 42, no. 6/7, April 1986, pp. 230-8
(D3) xe2x80x9cA higher order topology for interpolative modulators for oversampling A/D convertersxe2x80x9d, by Kirk C. H. Chao et al in IEEE Trans. on Circuits and Systems, Vol. 37, no. 3, March 1990, pp. 309-18
(D4) xe2x80x9cA method for the construction of minimum-redundancy codesxe2x80x9d, by D. A. Huffman in Proc. of the IRE, Vol. 40(10), September 1952.
(D5) xe2x80x9cAn introduction to arithmetic codingxe2x80x9d by G. G. Langdon, IBM J. Res. Develop., Vol. 28(2), March 1984.
(D6) xe2x80x9cA universal algorithm for sequential data compressionxe2x80x9d by J. Ziv et al, IEEE Trans. on Inform. Theory, Vol. IT-23, 1977.
The above citations are hereby incorporated in whole by reference.
The invention aims at providing a data compression apparatus for data compressing an audio signal such that it is more simple and that the corresponding expander apparatus can also be more simple and less expensive.
The data compression apparatus in accordance with the invention includes
input apparatus for receiving the audio signal,
conversion apparatus for carrying out a conversion on the audio signal so as to obtain a 1-bit bitstream signal, the conversion apparatus including sigma-delta modulator apparatus,
lossless coding apparatus for carrying out a substantially lossless data compression step on the bitstream signal so as to obtain a data compressed bitstream signal, and
output apparatus for supplying the data compressed bitstream signal. More specifically, when the audio signal is an analog audio signal, the conversion apparatus is in the form of A/D conversion apparatus for carrying out a 1-bit A/D conversion on the analog audio signal so as to obtain the bitstream signal.
The invention is based on the following recognition. The audio signal can be applied in analog form or in digital form. When A/D converting, in accordance with the invention, an analog audio signal with a 1-bit A/D converter (also named: bitstream converter or sigma-delta modulator), the audio signal to be A/D converted is sampled with a frequency which is generally a multiplicity of the frequency of 44.1 kHz or 48 kHz. The output signal of the 1-bit A/D converter is a binary signal, named bitstream signal. When the audio signal is supplied in digital form, sampled at eg. 44.1 kHz, the samples being expressed in eg. 16 bits per sample, this digital audio signal is oversampled with a frequency which is again a multiplicity of this sampling frequency of 44.1 kHz (or 48 kHz), which results in the 1-bit bitstream signal.
Converting an audio signal into a 1-bit bitstream signal has a number of advantages. Bitstream conversion is a high quality encoding method, with the possibility of a high quality decoding or a low quality decoding with the further advantage of a simpler decoding circuit. Reference is made in this respect to the publications xe2x80x98A digital decimating filter for analog-to-digital conversion of hi-fi audio signalsxe2x80x99, by J. J. van der Kam, document D2, and xe2x80x98A higher order topology for interpolative modulators for oversampling A/D convertersxe2x80x99, by Kirk C. H. Chao et al, document D3 in the list of related documents.
1-bit D/A converters are used in CD players, as an example, to reconvert the bitstream audio signal into an analog audio signal. The audio signal recorded on a CD disk is however not a data compressed 1-bit bitstream signal.
It is well known in the art that the resulting bitstream signal of the 1-bit A/D converter is, roughly, a random signal which has a xe2x80x98noisy-likexe2x80x99 frequency spectrum. Such types of signals are hard to data compress.
Surprisingly, however, it was established that using a lossless coder, such as a variable length coder in the form of a Huffman coder or an arithmetic coder, a significant data reduction could be obtained, in spite of the noisy character of the bitstream signal from the 1-bit A/D converter.